The present invention relates to a wheel suspension system for the driven rear wheels of a motor vehicle in a steerable and non-steerable arrangement and, more particularly, to a wheel suspension system comprising a wheel carrier which is supported by way of a spring strut on the vehicle body side and which is guided by way of two individual links forming an upper pivotal connection and by way of a lower pivotal connection consisting of an A-arm with a tie rod link which extends approximately in the transverse direction of the vehicle. The pivotal connections are supported on the vehicle body side in joints of different radial characteristics, and the individual links are arranged in a diverging manner in the vehicle transverse direction and are disposed in two spaced joints of the wheel carrier. The A-arm is held by a joint on the wheel carrier, and both pivotal connections form an elastokinematic wheel swivelling axis, which is disposed on the outer side of the wheel, with a steering pole in the wheel contact plane which is disposed, with respect to the driving direction, behind a perpendicular wheel center transverse plane and outside the perpendicular wheel center longitudinal plane.
A twin control arm suspension for unsteered wheels is described in a published report entitled, On the Sixth International Technical Conference on Experimental Safety Vehicles; Oct. 12-15, 1976, Pages 656 to 664) which comprises a lower A arm and an upper opened up control arm with two individual links, adjacent to which another link is assigned. The pivotal connections, which are arranged in superimposed horizontal planes, together form an elastokinematic swivelling axis for a wheel position change when longitudinal and lateral forces affect the wheel. For this purpose, the swivelling axis is aligned in such a manner that a wheel steering pole is formed in the wheel contact plane which is situated outside the wheel track and, with respect to the driving direction, behind a perpendicular wheel center transverse plane.
Furthermore, German Patent Document DE-PS 19 38 850 describes a wheel suspension system for steered front wheels comprising a lower A-arm and an upper opened-up control arm. The connecting lines of the intersecting points of the longitudinal axes of these suspension links situated in horizontal planes arranged above one another form an elastokinematic swivelling axis of the wheel.
Furthermore, from German Patent Document DE-OS 36 42 421 describes a wheel suspension for unsteered wheels of a motor vehicle which has a lower A-arm and an upper opened-up control arm as well as a tie rod. On the side of the vehicle body, the A-arm is held in longitudinally elastic bearings. The tie rod is supported in a joint and causes, when longitudinal and lateral forces act upon the wheel, a displacement of the A-arm in the longitudinal direction and, as a result, a wheel position change is triggered in the toe-in direction.
These known arrangements of suspension links with the forming elastokinematic swivelling axis of the wheel and an occurring wheel steering pole in the wheel contact plane cause wheel position changes in the driving operation. These are achieved only insufficiently by forces acting upon the wheel as well as during compression movements so that, on the whole, no stable vehicle handling is ensured along the entire speed range with the superposition of braking and starting or accelerating forces during cornering and straight-ahead driving.
Because of the pivotal connections to the vehicle body and a resulting setting and position with respect to the vehicle wheel, the effects of the known wheel suspensions are geared only to a special vehicle. A transfer of this known wheel suspension to a vehicle of a defined construction is not possible in a simple manner because of the different vehicle body structures and resulting changed pivotal connecting points on the vehicle body as well as their dimensions so that, when the driving conditions differ, a desired wheel position change is not optimally ensured.
The present invention is based on an object of providing a wheel suspension system in a twin control arm construction for steered and unsteered rear wheels of a motor vehicle which, on one hand, ensures a secure vehicle handling in all driving conditions with a good driving comport and, on the other hand, the wheel suspension system can be housed in the vehicle in a compact shape with respect to space.
According to the present invention, this object has been achieved by arranging the two pivotal connections, with respect to the driving direction, in inclined planes extending diagonally toward the front in a rising manner and are set at an angle with respect to one another. In these planes, by way of the vehicle-body-side joints of the two pivotal connections, spaced link rotating axes are formed which, in lateral view and viewed in the driving direction, are set at different angles with respect to the wheel contact surface and, in top view, are arranged to be crossed with respect to one another at an angle. The wheel-carrier-side joints of the two pivotal connections which are opposite the two link rotating axes, are situated close to a perpendicular wheel center transverse plane and directly adjacent to a perpendicular wheel center longitudinal plane. The vehicle-body-side joints of the two pivotal connections have a radial characteristic which is defined differently for the swivelling in the toe-in direction about the forward joint of the lower pivotal connection. Alternatively, between the tie rod links of each wheel side, an adjusting device for a rear-wheel steering is arranged which is connected with the free inner ends of each of the tie rod links which extend at an angle of almost 90.degree. with respect to the perpendicular wheel center longitudinal plane.
The two pivotal connections, in the present invention, are with respect to the driving direction arranged in inclined planes such that they rise diagonally toward the front and are set at an angle with respect to one another. In these planes, spaced link rotating axes are formed by the vehicle-body-side joints which, in the lateral view and viewed in the driving direction, are disposed at differently sized angles with respect to the wheel contact surface and, in the top view, are arranged crossed with respect to one another at an angle. The wheel-carrier-side joints of the two pivotal connections, which are opposite the two link rotating axes, are arranged close to a perpendicular wheel center transverse plane and directly adjacent to a perpendicular wheel center longitudinal plane. By way of the pivotal connections, an elastokinematic wheel swivelling axis is formed which, with respect to the driving direction, forms a steering pole in the wheel contact plane which is situated behind the vertical wheel center transverse plane and outside the perpendicular wheel center longitudinal plane. The vehicle-body-side joints of the pivotal connections have a radial characteristic which is defined for the elastic swivelling about the forward joint of the lower pivotal connection.
In the case of this arrangement and construction of the two pivotal connections as well as the defined characteristics of the joints, when longitudinal and laterall forces act upon the wheel, a targeted wheel position change is achieved in the toe-in direction about the joint of the lower A-arm, which is essentially caused by an elastokinematic swivelling axis with a piercing point in the wheel contact plane. This swivelling axis extends through intersecting points of the longitudinal axes of the links which are formed by the upper pivotal connection and the lower pivotal connection with the tie rod links. Because of the setting of the links of both pivotal connections, the swivelling axis has a position outside the perpendicular wheel center longitudinal plane with a piercing point (or wheel steering pole) outside the track width or of the perpendicular wheel center longitudinal plane and, with respect to the driving direction, behind the perpendicular wheel center transverse plane. By way of this arrangement of the wheel steering pole, an understeering tendency of the vehicle is achieved at the start of a cornering operation.
For the wheel adjustment under the effect of forces, the vehicle-body-side link bearings of the two pivotal connections have radial characteristics that are coordinated with one another in such a manner that the wheel can swivel virtually about the joint of the lower pivotal connection which is forward, with respect to the driving direction, and forms a pivot, and the rearward joint has for this purpose a much softer radial characteristic than the forward joint. Corresponding to the forward link of the lower pivotal connection, the tie rod link is supported in a radially hard manner on the vehicle body side. In the case of the upper pivotal connection, the forward individual link is supported in a harder manner on the vehicle body side than the rearward individual link. The tie rod link is constructed and disposed such that a significant proportion of the force, in the case of lateral force, is absorbed by this link.
The position of the elastokinematic swivelling axis is essentially determining for the desired wheel position changes in any driving condition. In order to achieve a favorable influencing of the handling of the motor vehicle, the two pivotal connections or the relevant links must be set correspondingly with respect to one another or the joints on the wheel carrier and on the vehicle body must have a corresponding position. Thus, it is advantageous for the piercing point (steering point) of the swivelling axis in the wheel contact plane to be situated at a relatively large distance from the perpendicular wheel center longitudinal and transverse plane. This is achieved when the swivelling axis, in the lateral view, extends from the steering pole situated behind the perpendicular wheel center transverse plane diagonally forward and intersects with the perpendicular wheel center transverse plane approximately at the level of the wheel-carrier-side joint of the rearward individual link of the upper pivotal connection. Another contributing factor is that the wheel-carrier-side joint of the A-arm is arranged closer to the perpendicular wheel center longitudinal plane than the corresponding joint of the tie rod link. The position of the individual links of the upper pivotal connection as well as the position of the wheel-carrier-side joint of the two individual links of the upper pivotal connection also contribute significantly to the fixing of the elastokinematic swivelling axis.
In the case of unsteered rear wheels, the tie rod links of the lower pivotal connection of both wheel sides are supported on the vehicle body or on a subframe. In contrast, in the case of steered rear wheels, these tie rod links are connected with an interposed control cylinder. As a function of the respective condition of the motor vehicle, this control cylinder causes a steering wheel adjustment.
Another swivelling axis, specifically the kinematic swivelling axis which acts particularly in the case of steerable rear wheels, is formed by the arrangement of the links of the upper and lower pivotal connections with respect to one another as well as by the position of the joints on the vehicle body as well as on the wheel carrier. Therefore, the kinematic swivelling axis extends through intersecting points formed by the longitudinal axes of the links, in a lateral view, from the wheel contact surface diagonally to the upward rear against the driving direction. Furthermore, this kinematic swivelling axis is arranged in front of the perpendicular wheel center transverse plane. As a result, a positive caster angle is formed so that, during the steering, the wheel that is on the outside during cornering can go into a negative camber, which is an advantage for increasing the maximally possible transmissible lateral force. In addition, as the result of the position of the kinematic swivel axis, a negative disturbing-force lever arm is obtained so that elastokinematics are achieved that are as favorable as possible, and a correction of the toe-in value can take place, for example, in the case of a load change, as well as during braking.
The rear axle according to the invention is constructed such that solely by way of the linking of the tie rod link to a control cylinder, a use as a steerable rear axle is achieved without the requirement of changing the geometry of the upper and lower pivotal connection. So that a more favorable lever arm is achieved for the steering, the tie rod link may be transferred from its course, with respect to the driving direction, from the front outside to the rear inside to the vehicle longitudinal center axis into a position where the tie rod link is arranged at an angle of almost 90.degree. with respect to the perpendicular wheel center longitudinal plane. As a result, a larger lever arm is obtained between the perpendicular wheel center transverse plane and the tie rod link than in the case of a setting of the tie red link when the rear wheels are not steered.
In particular, the tie rod link is arranged approximately in the same horizontal plane as the wheel-carrier-side joint of the A-arm and is held in the joint of an inwardly projecting arm of the wheel carrier, in which case the joint is arranged between the vehicle-body-side and the wheel-carrier-side joints of the two pivotal connections. As a result of this position of the joint, which is assigned to the wheel, on the arm of the wheel carrier, the spring strut can at the same time be supported by its lower end, in which case the tie rod link and the spring strut are held on a common axis.
The link rotating axes of the two pivotal connections formed by the vehicle-body-side joints are arranged in such a manner that one link rotating axis of the upper pivotal connection is arranged closer to the perpendicular longitudinal center plane of the wheel than the other link rotating axis of the lower pivotal connection. Both axes of rotation are, in the top view and viewed in the driving direction, arranged to be extending diagonally from the outside toward the inside front to the vehicle longitudinal center axis and at an acute opening angle with respect to one another. This slight crossing or the link rotating axes of the two pivotal connections with respect to one another is configured, in the case of compression movements, for a slight toe-in change. The upper and the lower pivotal connection are set in the diagonal planes to be extending by the link rotating axes at an opening angle with respect to one another, viewed in the driving direction, to the front interior in such a manner that an antisquat is increased by way of the compression path of the vehicle in the case of driving forces and load changes. This effect is achieved by the movement of the wheel, which is held by way of the two pivotal connections, with its wheel spin axis during the compression on a progressive curved path and, rises toward the rear, with respect to the driving direction, and the wheel spin axis moves on this curved path. The antisquat which rises by way of the compression path changes, by way of the positive and negative output forces (acceleration and load change), the rolling moment distribution of the front axle and the rear axle. During acceleration, the rear axle of the vehicle takes over a higher proportion of the rolling torque distribution; during a load change this proportion becomes smaller which leads to a tendency to understeer during a load change.
Furthermore, the link rotating axes extend in diagonal planes with respect to one another in such a manner that a pitching pole is obtained which, with respect to the driving direction, is situated in front of the wheel spin axis. By virtue of this position of the pitching pole, which forms because of the position of the upper and the lower pivotal connection as well as their joints with respect to one another, a linear antidive effect is achieved by way of the spring path. Thus, during a braking, the wheel contact point can move in a straight line toward the rear at an angle with respect to the wheel contact plane. As a result, a compromise is advantageously achieved between the tendency to understeer during the braking and a rebounding of the rear axle that is as low as possible.
In the lateral view, the spring strut is arranged approximately in parallel with respect to the perpendicular wheel center transverse plane and, with respect to the driving direction, is supported behind this plane on the wheel carrier. Furthermore, the spring strut extends between the tie rod link and the lower A-arm, on one side, and between the two upper individual links, on the other side, and is set approximately perpendicularly with respect to the wheel contact plane. As a result, in the interaction of the two pivotal connections, a favorable spring ratio is achieved because the lower pivotal connection of the spring strut is arranged relatively close to the wheel. The perpendicular position of the spring strut, in the lateral view, as well as the arrangement between the lower wishbone and the tie rod link results, during suspension operations, advantageously in no wheel position changed in the toe-in and toe-out direction.
The tie rod link is situated approximately in a horizontal plane with the lower pivotal connection so that the wheel suspension system according to the invention is also well suitable for a rear engine and all-wheel steering. By way of its pivotal connections, the present wheel suspension system according to the invention may be supported by way of joints on a subframe as well as on the vehicle body. The subframe is supported on the vehicle body by way of elastic elements.
Because of its arrangement, the A-arm is stressed only with respect to tenslon and pressure so that a connecting strut between the links is not necessary and a so-called open A-arm is used which has a light construction.